A programmable logic controller (PLC) is constituted by having incorporated therein a conversion device (hereinafter, “D/A conversion device”) that converts a digital value generated within the PLC into an analog value to input the analog value to a controlled device. As a conventional D/A conversion device, there is one type of D/A conversion device that consecutively D/A converts a digital value when the digital value is written from outside of the D/A conversion device (for example, a CPU device constituting the PLC) for each predetermined period. Even when the conventional D/A conversion device has a capability of quickly performing D/A conversion, because an actual D/A conversion rate depends on a rate at which a digital value is written from outside, there is problem in that the actual D/A conversion rate is decreased.
In this connection, for example, Patent Literature 1 discloses an analog-signal processing device for a programmable logic controller that performs a series of operations from A/D conversion to D/A conversion based on an analog signal input from outside without using any CPU device.